This invention relates to an image sensing apparatus such as a video camera or digital still camera and, more particular, to self-photography in which the photographer faces the apparatus to shoot video of him/herself.
The construction and operation of an image sensing apparatus according to the prior art will be described with reference to the block diagram of FIG. 16.
The image sensing apparatus shown in FIG. 16 includes a lens group 1 for forming the image of a subject. The lens group 1 constructs a zoom lens having a stationary first lens group 101, a variable power lens 102 for varying power, a stationary second lens group 103 and a focus compensating lens (referred to as a “focusing lens” below) 104 having a function for correcting movement of the focal plane that accompanies zooming and a function for focusing. A zoom lens motor 3 drives the variable power lens 102 of the zoom lens 1 to decide the zoom position, and a variable power lens drive unit 7 drives the variable lens motor 3. A zoom lens drive unit 4 drives the zoom lens motor 3. A focusing lens motor 7 drives the focusing lens 104 of the zoom lens 1 to perform focusing, and a focusing lens drive unit 8 drives the focusing motor 7. A diaphragm mechanism 2 such as a diaphragm blade mechanism controls the amount of incident light, a diaphragm mechanism drive motor 5 drives the diaphragm mechanism 2, and a diaphragm mechanism drive unit 6 drives the diaphragm mechanism drive motor 5. An image sensing device 9 converts incident light to an electric signal. A CDS/AGC unit 10 performs automatic gain control (referred to as “AGC” below) for sampling the signal obtained from the conversion by the image sensing device 9 and amplifying the signal electrically. An analog-digital converter (referred to as an “A/D converter” below) 11 converts the analog signal output of the CDS/AGC unit 10 to a digital signal. A camera signal processor 12 generates a standard television signal by subjecting the signal from the A/D converter 11 to processing such as a gamma correction, color separation and color-difference matrix. A digital/analog converter (referred to as a “D/A converter” below) 13 converts the output signal of the camera signal processor 12 from a digital to an analog signal. A monitor 14 is for checking the video of the subject photographed by the photographer, an AE evaluation value processor 15 obtains an evaluation value, which is for exposure control, from the video signal produced by the camera signal processor 12, and an AF value evaluation processor 16 obtains an evaluation value, which is for performing focusing control, from the video signal produced by the camera signal processor 12. A microcomputer 19 executes the processing of an exposure controller for controlling exposure by controlling the diaphragm mechanism 2 and the gain of the CDS/AGC unit 10 and the processing of focus controller for focusing the image of the subject, which is formed on the surface of the image sensing device, by controlling the focusing lens 104.
The image sensing apparatus having the construction described above is equipped with an image sensing assist mechanism that makes it possible to obtain the optimum medium video automatically or through a simple photographic operation. It should be noted that the term “automatic” is used here and in the claims to include the meaning of a semiautomatic operation “to obtain the optimum medium video through a simple photographic operation”. The image sensing assist mechanism includes an automatic exposure control unit (referred to as an “AE control unit” below) for automatically optimizing the constantly changing state of brightness of the subject being photographed, and an auto-focus control unit (referred to as an “AF control unit” below) for automatically focusing on the subject. These units will be described next.
The AE control unit will be described first.
The AE control unit uses a known method for sensing exposure based upon the video signal representing the subject, adjusting the amount of light incident upon the image sensing device 9 by the diaphragm mechanism 2 so as to optimize exposure, and controlling the gain of the video signal by the AGC means of the CDS/AGC unit 10 so that any change in brightness can be corrected for automatically.
The AE control in accordance with the arrangement of FIG. 16 is performed by the AE control unit of the microcomputer 19 in accordance with a control flowchart shown in FIG. 17.
Operation will now be described in accordance with the control flowchart of FIG. 17.
The AE evaluation value obtained by the AE evaluation value processor 15 is detected at step S901. The AE evaluation value is obtained by sensing, from the video signal produced by the camera signal processor 12, a change in the brightness of the subject. The AE evaluation value processor 15 senses exposure over the full area of the video, which impinges on the image sensing device 9, so as to optimize the exposure of the subject being photographed under a variety of photographic conditions.
Next, at step S902, the current state of exposure based upon the AE evaluation value detected at step S901 is compared with a reference value representing a preset exposure state considered to be optimum, it is determined whether the current state of exposure is optimum or not and, if exposure is improper, the amount of error with respect to the reference value is detected. If the proper exposure has been attained (“YES” at step S902), the current exposure control value is output as is. If exposure is improper (“NO” at step S902), then the diaphragm mechanism 2 or CDS/AGC unit 11 is controlled in conformity with the amount of error and a control value that will cause the current state of exposure to become the reference value is calculated. This will be described taking as an example a case in which exposure control is carried out in accordance with the program diagram shown in FIG. 18.
Consider photography in sufficient illumination such as outdoors (area B in FIG. 18). If AGC is fixed at a low gain (0 dB in FIG. 18) at step S906 and the brightness of the subject changes, a control value by which the diaphragm mechanism 2 compensates for the amount of error detected at step S902 is calculated and the iris of the diaphragm mechanism 2 is opened or closed to regulate the amount of light that impinges upon the image sensing device 9, thereby controlling the state of exposure in optimum fashion. If the subject darkens, the diaphragm mechanism 2 is opened fully (area A in FIG. 18) and exposure control can no longer be performed by the diaphragm mechanism 2, then the diaphragm mechanism is fixed in the fully open state at step S904. If the brightness of the subject then changes, then a control value by which the amount of error detected at step S902 is compensated for by AGC gain is calculated and the state of exposure is controlled for optimization by AGC gain.
The thus obtained control value of the diaphragm mechanism 2 or CDS/AGC unit 10 is output at step S907 to update the control value, whereby control for optimizing state of exposure is performed while following up brightness of the subject at all times.
The AF control unit will be described next.
The AF control unit uses a known method for sensing the sharpness of the image of the subject from the video signal and controlling the focusing lens position so as to maximize sharpness, thereby achieving focusing. In general, evaluation of sharpness is performed using the intensity of the high-frequency component of the video signal extracted from a bandpass filter or the blurring range detection intensity of the video signal extracted by a differentiating circuit or the like. If the subject has been photographed, these signals usually are small if the subject is out of focus and increase as focusing improves, as illustrated in FIG. 19. The maximum value is attained when the subject is brought into perfect focus. Accordingly, the focusing lens is controlled in such a manner that when the sharpness signal is small, the lens is moved rapidly in a direction that will enlarge the signal. As the signal increases, the lens is moved slowly and is halted precisely at the peak of the curve to obtain a focused image. The method of automatic focusing generally is referred to as the hill climbing method (referred as “hill-climbing AF” below).
Hill-climbing AF control in accordance with the arrangement of FIG. 16 will be described with reference to the control flowcharts of FIGS. 20 through 23 the processing of which is performed by the AF control unit of the microcomputer 19.
The AF evaluation value is acquired from the AF value evaluation processor 16 at step S1201 in FIG. 20 and the currently prevailing AF mode is discriminated at step S1202.
In restart discrimination processing, a value held at transition to a restart discrimination mode is compared with the AF evaluation value at step S1301 in FIG. 21. If it is found at step S1302 that the difference between the held value and the AF evaluation value is greater than a predetermined value, a transition is made to direction discrimination processing. If the difference is larger than the predetermined value, then processing is terminated.
In direction discrimination processing, wobbling is performed at step S1401 in FIG. 22 to oscillate the focusing lens 104 back and forth, thereby determining whether the image is in or out of focus and, if out of focus, the direction in which the lens should be moved to obtain the focused state. If the image is out of focus, control returns to the hill-climbing processing mode at step S1406. If the image is in focus, then control returns to the restart discrimination mode at step S1404.
In hill-climbing processing, the AF evaluation value is subjected to a peak holding operation at step S1501 in FIG. 23. Specifically, if the AF evaluation value is greater than the current peak value, then this value is adopted as the new peak value. If it is greater than the focusing lens position, this value is adopted as the new peak value and this focusing lens position is held. Next, it is determined at step S1502 whether the AF evaluation value is greater than the initial hill-climbing value held at step S1407 in FIG. 22. Processing is terminated if the AF evaluation value is not less than the peak value. If the AF evaluation is less than the peak value, then processing for returning to the focusing lens position of the peak value is executed at step S1504 and then it is determined at step S1505 whether the position has returned to the focusing ring position of the peak value. Control shifts to direction discrimination processing at step S1506 if the position has returned to the focusing ring position of the peak value; otherwise, processing is terminated. If it is found at step S1502 that the AF evaluation value is less than the initial hill-climbing value, then the hill-climbing direction is reversed and the focusing lens is driven in the opposite direction. This is followed by step S1508, at which the initial hill-climbing value is updated to the current AF evaluation value.
Thus, control is performed to move the focusing lens and maximize the AF evaluation value while repeating restart discrimination processing, direction discrimination processing and hill-climbing processing, whereby video in which the subject is in focus at all times is obtained.
In an image sensing apparatus such as a video camera having a photography assist mechanism such as the AE control unit and AF control unit, the conventional practice is to use an electronic viewfinder (referred to as an “EVF” below), which enlarges the video of a small-size CRT or liquid crystal panel by an optical enlargement unit such as an enlarging lens, as a monitor for verifying the video being captured by the photographer. Typically, the photographer shoots a scene while observing video that appears in the EVF by way of a configuration in which the display screen of the EVF is arranged to point in a direction opposite that of the lens, as illustrated in FIG. 24A.
An image sensing apparatus proposed in the recent past has a structure of the kind shown in FIG. 24B, in which the apparatus is provided with a monitor (referred to as a “large-size monitor” below) that allows the photographer to verify video by direct viewing using a large-size liquid crystal panel without relying upon an optical enlargement unit, or of the kind shown in FIG. 24C, in which the apparatus is provided with both an EVF and a large-size monitor.
With the image sensing apparatus equipped with the large-size monitor, ordinarily photography is performed in a state in which the display screen of the large-size monitor is faced in a direction opposite that in which light from the subject impinges upon the image sensing lens, as shown in FIG. 25A. However, unlike the EVF, the large-size monitor allows the photographer to verify the video on the monitor even if the photographer does not look directly at the screen. Accordingly, there has been proposed a structure that makes possible so-called self-photography. Specifically, as shown in FIG. 25B, the display screen of the large-size monitor is pointed in a direction the same as that in which light from the subject impinges upon the image sensing lens, thereby allowing the photographer to shoot video of himself or herself while observing the image on the large-size monitor, as illustrated in FIG. 26.
When self-photography is performed, AE, AF control that follows up the subject is performed by the AE control unit and AF control unit in the same manner as when ordinary photography is carried out, thereby providing the optimum video.
Control of the photography assist mechanism such as the AE control unit and AF control unit is control premised on the photographic conditions that prevail during ordinary photography. In self-photography, however, proper control is not always performed because the photographic conditions differ from those of ordinary photography.
For example, at the time of ordinary photography, the AE control unit achieves a certain degree of optimization in a variety of brightness conditions by taking into consideration not only scenes in which the entire picture has a uniform brightness but also back lighted scenes and scenes illuminated by spotlight, and control is performed quickly in response to a change in the brightness of the subject. With self-photography, however, the photographer shoots video of himself or herself while observing the video on the large-size monitor. Consequently, there is little change in the brightness of the main subject. When AE control similar to that at the time of ordinary photograph is performed, the camera reacts too sensitively to changes in the brightness of peripheral subjects, thereby causing a change in the brightness of the main subject.
In regard to the AF control unit, control at the time of ordinary photography is carried out so as to achieve focusing from close-up photography, in which the distance to the subject is less than one meter, to photography in which the subject distance is near infinity, as in the case of a landscape. Ina case where a nearby subject is present within an AF photometry area used to detect the AF evaluation value, the camera will focus on the main subject if the main subject is photographed predominantly at the center of the screen, as shown in FIG. 15A. However, if the camera is erroneously focused on the background in a case where the main subject has shifted from the center of the screen because of hand movement of the photographer or temporary movement of the main subject at the time of photography, as shown in FIG. 15B, the main subject at the time of self-photography will become much more blurred than at the time of ordinary photography because the distance to the main subject is approximately one meter.
Thus, when control of the photography assist mechanism such as the AE and AF units for dealing with a wide range of photographic conditions is performed in the same manner as at the time of ordinary photography when self-photography is carried out using an image sensing apparatus having a large-size monitor, often the video obtained is not that intended by the photographer.